[PATCH v5 3/6] usage: add show_usage_if_asked()

[Junio C Hamano <gitster@pobox.com>]

Some commands call usage() when they are asked to give the help
message with "git cmd -h", but this has the same problem as we
fixed with callers of usage_with_options() for the same purpose.

Introduce a helper function that captures the common pattern

	if (argc == 2 && !strcmp(argv[1], "-h"))
		usage(usage);

and replaces it with

	show_usage_if_asked(argc, argv, usage);

to help correct these code paths.

Note that this helper function still exits with status 129, and
t0012 insists on it.  After converting all the mistaken callers of
usage_with_options() to call this new helper, we may want to address
it---the end user is asking us to give the help text, and we are
doing exactly as asked, so there is no reason to exit with non-zero
status.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 git-compat-util.h |  2 ++
 usage.c           | 27 ++++++++++++++++++++++++---
 2 files changed, 26 insertions(+), 3 deletions(-)

diff --git a/git-compat-util.h b/git-compat-util.h
index e283c46c6f..d43dd248c4 100644
--- a/git-compat-util.h
+++ b/git-compat-util.h
@@ -701,6 +701,8 @@ int error_errno(const char *err, ...) __attribute__((format (printf, 1, 2)));
 void warning(const char *err, ...) __attribute__((format (printf, 1, 2)));
 void warning_errno(const char *err, ...) __attribute__((format (printf, 1, 2)));
 
+void show_usage_if_asked(int ac, const char **av, const char *err);
+
 #ifndef NO_OPENSSL
 #ifdef APPLE_COMMON_CRYPTO
 #include "compat/apple-common-crypto.h"
diff --git a/usage.c b/usage.c
index 47709006c1..38b46bbbfe 100644
--- a/usage.c
+++ b/usage.c
@@ -8,7 +8,7 @@
 #include "gettext.h"
 #include "trace2.h"
 
-static void vreportf(const char *prefix, const char *err, va_list params)
+static void vfreportf(FILE *f, const char *prefix, const char *err, va_list params)
 {
 	char msg[4096];
 	char *p, *pend = msg + sizeof(msg);
@@ -32,8 +32,13 @@ static void vreportf(const char *prefix, const char *err, va_list params)
 	}
 
 	*(p++) = '\n'; /* we no longer need a NUL */
-	fflush(stderr);
-	write_in_full(2, msg, p - msg);
+	fflush(f);
+	write_in_full(fileno(f), msg, p - msg);
+}
+
+static void vreportf(const char *prefix, const char *err, va_list params)
+{
+	vfreportf(stderr, prefix, err, params);
 }
 
 static NORETURN void usage_builtin(const char *err, va_list params)
@@ -173,6 +178,22 @@ void NORETURN usage(const char *err)
 	usagef("%s", err);
 }
 
+static void show_usage_if_asked_helper(const char *err, ...)
+{
+	va_list params;
+
+	va_start(params, err);
+	vfreportf(stdout, _("usage: "), err, params);
+	va_end(params);
+	exit(129);
+}
+
+void show_usage_if_asked(int ac, const char **av, const char *err)
+{
+	if (ac == 2 && !strcmp(av[1], "-h"))
+		show_usage_if_asked_helper(err);
+}
+
 void NORETURN die(const char *err, ...)
 {
 	va_list params;
-- 
2.48.1-218-gc7e8be6a8f